The present invention relates to an improved microwave-interactive cooking package. In particular, the present invention relates to high efficiency, safe and abuse-tolerant susceptor and foil materials for packaging and cooking microwavable food.
Although microwave ovens have become extremely popular, they are still seen as having less than ideal cooking characteristics. For example, food cooked in a microwave oven generally does not exhibit the texture, browning, or crispness which are acquired when food is cooked in a conventional oven.
A good deal of work has been done in creating materials or utensils that permit food to be cooked in a microwave oven to obtain cooking results similar to that of conventional ovens. The most popular device being used at present is a plain susceptor material, which is an extremely thin (generally 60 to 100 .ANG.) metallized film that heats under the influence of a microwave field. Various plain susceptors (typically aluminum, but many variants exist) and various patterned susceptors (including square matrix, "shower flower", hexagonal, slot matrix and "fuse" structures) are generally safe for microwave cooking. However, susceptors do not have a strong ability to modify a non-uniform microwave heating pattern in food through shielding and redistributing microwave power. The quasi-continuous electrical nature of these materials prevents large induced currents (so limiting their power reflection capabilities) or high electromagnetic (E-field) strengths along their boundaries or edges. Therefore their ability to obtain uniform cooking results in a microwave oven is quite limited.
Electrically "thick" metallic materials (e.g., foil materials) have also been used for enhancing the shielding and heating of food cooked in a microwave oven. Foil materials are much thicker layers of metal than the thin metallized films of susceptors. Foil materials, also often aluminum, are quite effective in the prevention of local overheating or hot spots in food cooked in a microwave by redistributing the heating effect and creating surface browning and crisping in the food cooked with microwave energy. However, many designs fail to meet the normal consumer safety requirements by either causing fires, or creating arcing as a result of improper design or misuse of the material.
The reason for such safety problems is that any bulk metallic substance can carry very high induced electric currents in opposition to an applied high electromagnetic field under microwave oven cooking. This results in the potential for very high induced electromagnetic field strengths across any current discontinuity (e.g., across open circuit joints or between the package and the wall of the oven). The larger the size of the bulk metallic materials used in the package, the higher the potential induced current and induced voltage generated along the periphery of the metallic substance metal. The applied E-field strength in a domestic microwave oven might be as high as 15 kV/m under no load or light load operation. The threat of voltage breakdown in the substrates of food packages as well as the threat of overheating due to localized high current density may cause various safety failures. These concerns limit the commercialization of bulk foil materials in food packaging.
Commonly owned Canadian Patent No. 2196154 offers a means of avoiding abuse risks with aluminum foil patterns. The structure disclosed addresses the problems associated with bulk foil materials by reducing the physical size of each metallic element in the material. Neither voltage breakdown, nor current overheat will occur with this structure in most microwave ovens, even under abuse cooking conditions. Abuse cooking conditions can include any use of a material contrary to its intended purpose including cooking with cut or folded material, or cooking without the intended food load on the material. In addition, the heating effectiveness of these metallic materials is maximized through dielectric loading of the gaps between each small element which causes the foil pattern to act as a resonant loop (albeit at a much lower Q-factor (quality factor than the solid loop). These foil patterns were effective for surface heating. However, it was not recognized that a properly designed metallic strip pattern could also act to effectively shield microwave energy to further promote uniform cooking.
Commonly owned U.S. patent application Ser. No. 08/037,909 approaches the problem differently by creating low Q-factor resonant circuits by patterning a susceptor substrate. The low Q-factor operation described in U.S. patent application Ser. No. 08/037,909 provides only a limited degree of power balancing.